This invention relates generally to oxygen sensing methods and apparatus, and, more particularly, relates to apparatus and methods for measuring oxygen concentration in gas mixtures by magnetic means.
Accurate measurement of oxygen concentration in a gas mixtures is important in a wide range of industrial, clinical and laboratory processes. A wide range of devices have therefore been proposed or developed for measuring oxygen concentration. It has long been recognized that oxygen is paramagnetic, in that its molecules seek the strongest part of a magnetic field. Most other gases, in contrast, are diamagnetic, in that their molecules seek the weakest part of a magnetic field. The widely recognized paramagnetic properties of oxygen have stimulated a number of investigations into methods and apparatus for measuring oxygen concentration in gas mixture by magnetic sensing apparatus.
An early type of paramagnetic measuring cell, which relied upon the magnetic susceptibility of oxygen, is described in Pauling, et al, "An Instrument for Determining the Partial Pressure of Oxygen in a Gas", 68 Journal of the American Chemical Society 795, (1946). The Pauling et al measuring cell utilizes a sealed glass tube containing a weakly diamagnetic gas, such as nitrogen. The tube is suspended between the wedge-shaped pole pieces of a permanent magnet, which provide a non-uniform magnetic field, and the tube is free to rotate about a vertical axis. The entire structure is then placed within a chamber containing a selected gas.
When oxygen is introduced into the chamber surrounding the tube, the nitrogen in the tube is effectively diamagnetic relative to the surrounding paramagnetic oxygen gas, and the tube experiences a force tending to rotate it into the region where the magnetic field is weakest. This movement, or a force required to prevent this movement, can be measured as an indication of the concentration of oxygen in the chamber. The Pauling cell, however, is fragile, and the rotational axis of the tube must be consistently oriented for each use, rendering it unsuitable for industrial oxygen measurement applications.
Another type of apparatus for measuring the concentration of oxygen relies upon the inverse relationship between temperature and the magnetic susceptibility of oxygen. As a result of this inverse relationship, heating a portion of an oxygen-containing mixture in a non-homogeneous magnetic field creates a "magnetic wind" effect, which can be measured through its thermal effect on an electrical resistance element. Various configurations of magnetic wind devices are discussed in Medlock, et al, "Oxygen Analysis", Transactions of the Instruments and Methods Conference, Stockholm, 1949, pp. 1-8; and Ellis, et al, "The Measurement of Gaseous Oxygen Tension Utilizing Paramagnetism", 40 British Journal of Anaesthesia 569 (1968).
Conventional magnetic wind oxygen measurement devices, however, are subject to relatively large errors due to the changes in the thermal properties of the surrounding, or "background" gases. In particular, the presence of different background gases causes conventional magnetic wind oxygen sensors to yield false readings of oxygen levels, due to the large differences in thermal characteristics of the background gases. Additionally, conventional magnetic wind oxygen sensing devices suffer from position sensitivity and background gas dependency, especially in comparison with methods based on direct measurement of magnetic susceptibility.
Accordingly, there exists a need for oxygen sensing methods and apparatus which provide accurate and reliable measurement of oxygen concentration, regardless of the composition of the background gases.
It is accordingly an object of the invention to provide oxygen sensing methods and apparatus which yield accurate measurement of oxygen concentration, independent of background gas composition and thermal properties.
It is a further object of the invention to provide oxygen measurement apparatus which are rugged, reliable, and readily portable.
Other general and specific objects of the invention will in part be obvious and will in part appear hereinafter.